AB101 DSB Modulator and Demodulator
Operating Manual Ver.1.1
An ISO 9001 : 2000 company
94-101, Electronic Complex Pardesipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 e mail :
[email protected] Website : www.scientech.bz Toll free : 1800-103-5050
AB101
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AB101 DSB modulator and demodulator AB101 Table of Contents 1.
Introduction
4
2.
Theory
6
3.
Experiments •
Experiment 1 Study of double Sideband AM Generation
13
•
Experiment 2 Study of Double sideband AM Reception by Envelope Detector
17
4.
Warranty
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5.
List of Accessories
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RoHS Compliance
Scientech Products are RoHS Complied. RoHS Directive concerns with the restrictive use of Hazardous substances (Pb, Cd, Cr, Hg, Br compounds) in electric and electronic equipments. Scientech products are “Lead Free” and “Environment Friendly”. It is mandatory that service engineers use lead free solder wire and use the soldering irons upto (25 W) that reach a temperature of 450°C at the tip as the melting temperature of the unleaded solder is higher than the leaded solder. Scientech Technologies Pvt. Ltd.
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AB101 Introduction AB101 is a compact, ready to use DSB Modulator and Demodulator experiment board. This is useful for students to understand process of modulation and demodulation. Test points are provided in board to understand waveforms at inputs and outputs of each block. It can be used with Scientech Analog Lab ST2612 which has built in DC power supply, AC power supply, function generator, modulation generator, continuity tester, toggle switches, and potentiometer. List of Boards : Model
Name
AB01 AB02 AB03 AB04 AB05 AB06 AB07 AB08 AB09 AB10 AB11 AB12 AB13 AB15 AB14 AB16 AB17 AB18 AB19 AB20 AB21 AB22 AB23 AB25
Diode characteristics (Si, Zener, LED) Transistor characteristics (CB NPN) Transistor characteristics (CB PNP) Transistor characteristics (CE NPN) Transistor characteristics (CE PNP) Transistor characteristics (CC NPN) Transistor characteristics (CC PNP) FET characteristics Rectifier Circuits Wheatstone Bridge Maxwell’s Bridge De Sauty’s Bridge Schering Bridge Common Emitter Amplifier Darlington Pair Common Collector Amplifier Common Base Amplifier Cascode Amplifier RC-Coupled Amplifier Direct Coupled Amplifier Class A Amplifier Class B Amplifier (push pull emitter follower) Class C Tuned Amplifier Phase Locked Loop (FM Demodulator & Frequency Divider / Multiplier) Multivibrator ( Mono stable / Astable) F-V and V-F Converter V-I and I-V Converter Zener Voltage Regulator Transistor Series Voltage Regulator Transistor Shunt Voltage Regulator DC Ammeter Instrumentation Amplifier Differential Amplifier (Transistorized)
AB28 AB29 AB30 AB31 AB32 AB33 AB35 AB39 AB41
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AB101 AB42 AB43 AB44 AB45 AB49 AB51 AB52 AB54 AB56 AB57 AB58 AB59 AB64 AB65 AB66 AB67 AB68 AB80 AB82 AB83 AB84 AB85 AB88 AB89 AB90 AB91 AB92 AB93 AB96 AB97 AB101 AB102 AB106
Operational Amplifier (Inverting / Non-inverting / Differentiator) Operational Amplifier (Adder/Scalar) Operational Amplifier (Integrator/ Differentiator) Schmitt Trigger and Comparator K Derived Filter Active filters (Low Pass and High Pass) Active Band Pass Filter Tschebyscheff Filter Fiber Optic Analog Link Owen’s Bridge Anderson’s Bridge Maxwell’s Inductance Bridge RC – Coupled Amplifier with Feedback Phase Shift Oscillator Wien Bridge Oscillators Colpitt Oscillator Hartley Oscillator RLC Series and RLC Parallel Resonance Thevenin’s and Maximum power Transfer Theorem Reciprocity and Superposition Theorem Tellegen’s Theorem Norton’s theorem Diode Clipper Diode Clampers Two port network parameter Optical Transducer (Photovoltaic cell) Optical Transducer (Photoconductive cell/LDR) Optical Transducer (Phototransistor) Temperature Transducer (RTD & IC335) Temperature Transducer (Thermocouple) DSB Modulator and Demodulator SSB Modulator and Demodulator FM Modulator and Demodulator ………… and many more
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AB101 Theory Amplitude Modulation (AM) Amplitude modulation is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage. In short information signal (modulating signal) is used to control the amplitude of the carrier wave. As the information signal increases in amplitude, the carrier wave is also made to increase in amplitude. Likewise, as the information signal decreases, then the carrier amplitude decreases. A double sideband transmission was the first method of modulation developed and, for broadcast stations, is still the most popular. By looking at figure 1 below, two inputs are coming in amplitude modulator block one is the information signal i.e. modulating signal and the other is high frequency carrier signal we can see that the modulated carrier wave does appear to 'contain' in same way the information as well as the carrier.
Figure 1
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AB101 Depth of Modulation : The amount by which the amplitude of the carrier wave increases and decreases depends on the amplitude of the information signal and is called the 'depth of modulation'. The depth of modulation can be quoted as a fraction or as a percentage. Percentage modulation =
V max − V min V max + V min
x 100%
Here is an example:
Figure 2 In above figure 2 we can see that the modulated carrier wave varies from a maximum peak-to-peak value of 10 volts, down to a minimum value of 6 volts. Inserting these figures in the above formula, we get: Percentage modulation
=
10 − 6 10 + 6
=
x 100% 4
x 100 %
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= 25% or 0.25
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AB101 Frequency Spectrum : Assume a carrier frequency (fc) of 1 MHz and amplitude of, say 5 volts peak-to-peak. The carrier could be shown as :
Figure 3 If we also have a 1 KHz information signal, or modulating frequency (fm), with amplitude of 2V peak-to-peak it would look like this:
Figure 4 When both signals have passed through the amplitude modulator they are combined to produce an amplitude modulated wave. The resultant AM signal has a new frequency spectrum as shown in figure 5 inserting changes that occurs as a result of the modulation process : 1.
The original 1 KHz information frequency has disappeared.
2.
The 1 MHz carrier is still present and is unaltered.
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AB101 There are two new components :
Figure 5 Carrier frequency (fc) plus the information frequency, called the upper side frequency (fc +fm). And Carrier frequency (fc) minus the information frequency, called the lower side frequency (fc - fm). The resulting signal in this example has a maximum frequency of 1001 KHz and a minimum frequency of 999 KHz and so it occupies a range of 2 KHz. This is called the bandwidth of the signal. Notice how the bandwidth is twice the highest frequency contained in the information signal. If the information signal consisted of range of frequencies, each separate frequency will create its own upper side frequency and lower side frequency. The Power in Sidebands : The modulated carrier wave i.e. the DSB output contains the original carrier and the sidebands. The carrier wave is unaltered by the modulation process and contains at least two thirds of the total transmitted power. The remaining power is shared between the two sidebands. The power distribution depends on the depth of modulation used and is given by : Total power = (carrier power)
(1 + N 2 ) 2
Where N is the depth of modulation, the greater the depth of modulation, the greater is the contained within the sidebands. The highest usable depth of modulation is 100% (above this the distortion becomes excessive). Since, at least twice as much power is wasted as is used; this form of modulation is not very efficient when considered on a power basis.
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AB101 Double sideband (DSB) Modulator : We are using balanced modulator, which is based on IC MC1496, to modulate our information signal. In figure 6 we can see that the amplitude of the carrier is increased and decreased in sympathy with the incoming information signal.
AM Modulation Process Figure 6 To emphasize the connection between the information and the final waveform, a line is sometimes drawn to follow the peaks of the carrier wave as shown in figure 7. This shape, enclosed by a dashed line in out diagram, is referred to as an 'envelope', or a 'modulation envelope'.
Figure 7 The modulated signal is now nearly ready for transmission. If the modulation process has given rise to any unwanted frequency components then a band pass filter can be employed to remove them. Scientech Technologies Pvt. Ltd.
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AB101 Double sideband (DSB) Demodulator : We are using envelope detector i.e. diode detector to demodulate the modulated signal. Diode Detector : The function of the diode detector is to extract the audio signal from the modulated signal. It performs this task in a very similar way to a half wave rectifier converting an AC input to a DC output. Figure 8 shows a simple circuit diagram of the diode detector.
Figure 8 In figure 8, the diode conducts every time the input signal applied to its anode is more positive than the voltage on the top plate of the capacitor. When the voltage falls below the capacitor voltage, the diode ceases to conduct and the voltage across the capacitor leaks away until the next time the input signal is able to switch it on again. See figure 9.
Figure 9
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AB101 The result is an output which contains three components : 1.
The wanted audio information signal.
2.
Some ripple of the high frequency.
3.
A positive DC voltage level.
A low pass filter followed by the envelope detector on the board is used to remove the high frequency ripple and a capacitor blocks the DC voltage level. Figure 10 shows the result of the information signal passing through the diode detector and low pass filter.
The input to the diode detector
Output of diode detector includes: a DC level, the audio signal and ripples.
Output after filtering Figure 10 Again the information signal is getting at the output of the filter block. If you varied frequency of the information signal corresponding changes occurs in the output frequency of the filter.
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AB101 Experiment 1 Objective : Study of double Sideband AM Generation Equipments Needed : 1.
Analog board AB101
2.
DC power supplies +12V and -12V from external source or ST2612 Analog Lab.
3.
Oscilloscope
4.
Function generator (ST4063)
5.
2 mm patch cords
Figure 11
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AB101 Procedure : This experiment investigates the generation of double sideband amplitude modulated (AM) waveforms, using the AB101 board. •
Connect +12V and -12V DC power supplies at their indicated position from external source or ST2612 Analog Lab.
1.
With the help of function generator (ST4063 Function generator) set sine wave of 1 MHz frequency and amplitude 800mV or 0.8Vpp approximately this is the carrier input to our balanced modulator.
2.
Give carrier frequency input to carrier signal socket to the AB101 board.
3.
With the help of function generator (ST4063 Function generator) set sine wave of frequency ranging to 300 Hz to 3.4 KHz and amplitude 2Vpp approximately this is the audio frequency sine wave which will be as our modulating signal.
4.
Give information signal to modulating signal socket to the AB101 board.
5.
Turn on power to the DSB modulator and demodulator (AB101) board.
6.
Monitor, in turn, the two inputs to the balanced modulator block, at TP1 and TP2 with the help of oscilloscope.
7.
Turn the balance potentiometer P1, on the board. It is this block that we will use to perform double-side band amplitude modulation.
8.
Next, examine the output of the balanced modulator block at TP3, together with the modulating signal at TP1, with the help of oscilloscope. Trigger the oscilloscope on the TP 3 signal. Check that the waveforms as shown in figure 12.
Figure 12
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AB101 The output from the balanced modulator block (at TP3) is a double-sideband. AM waveform, which has been formed by amplitude-modulating the 1MHz carrier sine wave with the audio-frequency sine wave. The frequency spectrum of this AM waveform is as shown below in figure 13, where fm is the frequency of the audio modulating signal.
Figure 13 9.
To determine the depth of modulation, measure the maximum amplitude (V max) and the minimum amplitude (Vmin) of the AM waveform at t.p.3, and use the following formula : Percentage Modulation =
Vmax − Vmin Vmax + Vmin
Where V max and V min are the maximum and minimum amplitudes shown in figure 13. 10.
Now vary the amplitude and frequency of the modulating signal, by adjusting the amplitude and frequency knob of the function generator. Note the effect that varying each potentiometer has on the amplitude modulated waveform (TP3). The amplitude and frequency of the two sidebands can be reduced to zero by reducing the amplitude of the modulating audio signal to zero. Do this by turning the amplitude potentiometer to its MIN position, and note that the signal at t.p. 3 becomes an un-modulated sine wave of frequency 1 MHz, indicating that only the carrier component now remains. Return the amplitude potentiometer to its maximum position. 11. Now turn the balanced potentiometer P1 on the board, until the signal at TP3 is as shown in figure 14.
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AB101
Figure 14 The balance potentiometer varies the amount of the 1 MHz carrier component, which is passed from the modulator's output. By adjusting the potentiometer until the peaks of the waveform (A, B, C and so on) have the same amplitude, we are removing the carrier component altogether. We say that the carrier has been 'balanced out' (or 'suppressed') to leave only the two sidebands. This is called Double sideband suppressed carrier (DSBSC) modulation. Note also that the amplitude of the modulating signal can be reduced to zero, by turning the Amplitude Var. knob on the function generator.
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AB101 Experiment 2 Objective : To Study Double Sideband AM Reception by Envelope Detector Procedure : This experiment investigates the reception and demodulation of AM waveforms. Now the next step is to extract audio information from the amplitude modulated wave. This operation is performed by the diode detector block, whose output follows the changes in the amplitude of the signal at its input. 1.
Connect cord between DSB output and DSB input sockets. By this connection, we give modulated input to the envelope detector.
Figure 16 2.
Next set the output at TP3 and at TP1 according to the figure
3.
Observe the signal flow from the input of envelope detector to anode of diode at TP5 and on its cathode at TP6. Note that the signal at the diode detector's output follows the amplitude variations of the incoming signal as required.
4.
Vary the potentiometer P2 in the envelope detector block while observing the output of envelope detector.
5.
You can see variations in the detected output when you change the RC time constant of the filter formed by potentiometer P2 and capacitor C.
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AB101 6.
Next examine the output of filter at TP7 is changing when you change the frequency of information signal in the range 300 Hz to 3.4 KHz.
Figure 17
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AB101 Warranty 1.
We guarantee the product against all manufacturing defects for 24 months from the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.
2.
The guarantee will become void, if a)
The product is not operated as per the instruction given in the operating manual.
b)
The agreed payment terms and other conditions of sale are not followed.
c)
The customer resells the instrument to another party.
d)
Any attempt is made to service and modify the instrument.
3.
The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.
4.
The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.
For any Technical Problem Please Contact us at
[email protected]
List of Accessories 1.
2mm Patch Cord (Red) 16” ................................................................... 2 Nos.
2.
2mm Patch Cord (Black) 16”................................................................. 4 Nos.
3.
2mm Patch Cord (Blue) 16” .................................................................. 4 Nos.
4.
e-Manual.................................................................................................1 No. Updated 26-06-2009
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